Pause activities behind keyguard after boot.

Following boot the initial activity was automatically resumed even if
a lockscreen is obscuring it. Refer to CL 363859 for why this breaks
things.

This fix pauses all activities the first time a lockscreen appears.

Completes the fix for bug 10732489.

Change-Id: I6fcac14b574c495aa0e16d798cddc1263c6b4c25